Floods are no longer a concern only on large rivers. As a result of climate change, the likelihood of flash floods is increasing. Are cities and municipalities prepared for this?

Heavy rain can swell even harmless streams, flood roads and destroy homes within minutes.

What is treacherous is that the advance warning time for these storm events is often extremely short, in contrast to a river flood – despite better radar remote sensing and more refined forecast models by meteorologists. It is all the more important that the information chains become faster and smoother, says Thomas Kratzsch, head of the advisory and warning services department at the German Weather Service (DWD) in Offenbach.

Whether and which mistakes were made in the warning of the flood disaster in the Ahr Valley in 2021 is currently being politically and legally processed. At least 220 people died in Belgium and Germany as a result of flooding as a result of heavy rain last July. According to the DWD, heavy rain events are to be expected between May and September, most frequently in July.

An international team of scientists has calculated that climate change has increased the likelihood of extreme rainfall by a factor of 1.2 to 9. The focus of their study, which was published after the flood disaster, was on the regions around the Ahr and Erft and the region around the Maas river in Belgium.

Researcher: “It’s not enough”

Researchers complain that Germany is not sufficiently prepared for heavy rain events. “It’s not enough to put sirens on roofs and develop warning apps,” criticizes Christian Kuhlicke from the Helmholtz Center for Environmental Research (UFZ). It all starts with mapping: What dangers can occur where? With what consequences for the infrastructure, the buildings? Concrete measures such as retention basins would then have to be planned. Evacuation plans are also necessary.

A very large area of ​​Germany – the entire low mountain range region – is at risk from local heavy rain events, emphasizes the professor for environmental risks and sustainability at the University of Potsdam. In large cities, the focus must be on potentially life-threatening traps such as basements, underground car parks and subway shafts.

The recently presented study “Heavy rain and urban flash floods – Agenda 2030” by the Technical University of Kaiserslautern also calls for better protective measures. Theo Schmitt, one of the authors, demands that cities and municipalities should be obliged to carry out heavy rain risk management and draw up hazard maps.

Look at other countries

“Prevention has played too little of a role so far,” criticizes Jörg Asmussen, General Manager of the General Association of the German Insurance Industry (GDV). According to Asmussen, for example, most of the houses in the Ahr Valley are being rebuilt in their original locations. “We should be guided by countries like Switzerland, where building is not allowed in risk areas,” says the head of the association. Based on the damage reports, the GDV has been observing an increasing risk of heavy rain for years. At 8.2 billion euros, most of the 9.6 billion euros in natural hazards in 2021 as a whole was attributable to the flash flood in the summer, according to GDV. For comparison: in 2020, the elementary damage nationwide was 310 million euros.

After the storm disaster in July 2021, a team of scientists, including Kuhlicke from the UFZ, developed “Five principles for climate-proof municipalities and cities”. Among other things, the focus is on the critical infrastructure – the supply of water and electricity, hospitals and daycare centers must function even in extreme weather conditions, it is said. “Promoting the climate safety of buildings” is another point.

The German Association for Water, Wastewater and Waste (DWA) has been offering so-called heavy rain audits for municipalities since 2011. As a rule, a team of experts comes to a community for two days and brings all the stakeholders involved in prevention to one table. Cologne started eleven years ago, followed a little later by Braunschweig and Dresden – meanwhile 80 municipalities have taken advantage of the offer. Some federal states, such as Bavaria, provide financial support for the audits.

Regional association helpful

“A core problem is that we are often only addressed by municipalities that already have a strong risk awareness,” says Christian Siemon, who heads heavy rain audits for the DWA. “But the greatest need for action is usually where you are not aware of the existing risks,” says the civil engineer, whose office in Braunschweig specializes in flood prevention. Siemon thinks it makes sense for municipalities to join forces regionally and coordinate heavy rain prevention – for example if they are on the same stream or river.

According to experts, an important element of precaution is to design communities, cities and landscapes like sponges and to improve water retention in the landscape. A district for 3,500 people is currently being planned in Leipzig, which should be as well prepared as possible for extreme weather events.

Roland Müller is head of the “Leipzig BlueGreen” project. “The aim is for all rainwater to stay in the neighborhood,” explains the biotechnologist. The water is collected in troughs, containers and through green roofs, among other things. The concept of the sponge city is also about using stored water again, for example to keep inner courtyards and parks green all year round.

With new buildings, water-sensitive planning is easier than with existing renovations, says Müller. Especially with a view to increasing periods of drought, it makes sense for all cities to think about water storage. According to the project manager, the start of construction for the new district in Leipzig is expected to be mid to late 2023, which should be optimally prepared for high water, flooding, heavy rain and drought.